Ultrasound, as used for medical applications, utilizes high frequencies, typically between 1 and 30 MHz for imaging and flow measurements, and between 0.05 and 1.00 MHz for therapy, all of which are poorly transmitted by air and require a conduction medium similar in acoustic properties to tissue, commonly a thick fluid or gel. The ultrasound coupling gel or fluid displaces air and fills contours between the piezoelectric eye or transducer of an ultrasound instrument (such as a probe or scanhead), which converts energy between electrical and acoustic, and the body into which the sound is being directed. Examples of ultrasound probes or scanheads can be found in U.S. Pat. No. 5,482,047 to Nordgren et al. or U.S. Pat. No. 5,207,225 to Oaks et al. This gel or fluid material, by nature of its physical and acoustic properties, serves as an ultrasound acoustic coupler between the ultrasound transducer and tissue, thereby acoustically joining the two, so that the ultrasound based information developed can freely pass back and forth between the body and the transducer.
Because of the coupling effect, this media is commonly referred to as an ultrasound couplant, ultrasound gel, scanning gel, ultrasound transmission media or acoustic transmission media. Many fluids and water-based gels have been used as ultrasound couplants over the years. Early use of mineral oil was replaced by gels whose thickness was provided from a polymer group consisting of a copolymer of methyl vinyl ether, maleic anhydride, carboxy polymethylene polymer and mixtures thereof, or from a mixture of carboxy polymethylene polymer neutralized with an alkaline agent as a primary thickener together with hydroxy alkyl cellulose as an auxiliary thickener and a polyalkylene glycol such as propylene glycol as a humectant, as described in U.S. Pat. No. 4,002,221 to Buchalter and U.S. Pat. No. 4,459,854 to Richardson et al.
Biopsy, aspiration, amniocentesis and other puncture procedures guided with ultrasound, and ultrasound imaging procedures conducted during surgery currently use a combination of latex or synthetic elastomeric covers and forms of commercial fluids or gels external to the cover for coupling the ultrasound from the transducer to the patient. Hence, as a puncturing needle passes through the gel on the skin of the patient, minute quantities of the gel may be carried into the underlying tissue and the body cavity thereby introducing a likely tissue-incompatible substance into the patient.
Furthermore, since latex and synthetic elastomer covers in current use do not couple ultrasound adequately and uniformly over the entire active area of a transducer, a commercial coupling fluid or gel must also be inserted between the active surface of the transducer and the cover. Such covers containing commercial coupling fluids or gels occasionally rupture, tear, or are cut while being used for intraoperative, intracavity, or biopsy procedures, rendering the patient at risk from exposure to fluid or gel couplant chemicals introduced into the body, which are not typically biocompatible. In addition to an acoustic coupling fluid or gel between the transducer and the latex or synthetic elastomer cover, an additional acoustic coupling fluid, gel, or a lubricating jelly between the exterior of the cover and tissue is often required. The application of chemical ultrasound coupling fluids, gels and lubricating jelly, and the presence of latex, talc and associated compounds create documented problems of biological incompatibility which can be immediately life threatening as well as have severe long term consequences to the patient.
Fluids and gels commonly used as ultrasound couplants have several fundamental disadvantages, some of which are described below.
1. Fluids and gels offer no microbial barrier between the patient and the transducer; thus, latex rubber or synthetic elastomer probe covers must be applied over the transducer, to prevent transmission of microorganisms to the patient. In the prior art, two layers of couplant, one inside and one outside the cover, are required to provide ultrasound acoustic coupling between transducer and the patient. This potential infection concern is readily apparent when the transducer is used for imaging during needle biopsy or aspiration, or inside the body during surgery in direct organ, tissue and blood contact. Of growing importance is the protection from infection by skin transmission to patients who are immune compromised by disease, organ replacement, immune system modification, chemotherapy or radiation treatments. PA1 2. Fluids or gels are difficult to contain on, and remove from the patient during and after the ultrasound procedure, and introduce problems to the electronics by their chemically degrading nature. PA1 3. Commercially available oils and water based gels may react with the adhesives, elastomers, and epoxies used in the construction of medical ultrasound transducers, thus appreciably degrading performance and shortening their service life. PA1 4. Fluids or thickened water-based gels typically used in medical ultrasound, similarly described as in U.S. Pat. No. 4,002,221, are comprised of chemical compounds such as acrylic polymers, carboxy alkyl cellulose, hydroxyethylcellulose, carboxy polymethylene, organic acids, alkali metal salts, parabens and other germicidal and fungicidal agents, and surfactants. Such chemicals are not approved, or suitable for use in applications where they may be carried into the body, such as during biopsy, intra-operative procedures, or when the transducer is placed inside a body orifice. In instances where sterile latex rubber or synthetic covers containing thickened ultrasound coupling gels are used in surgery, tearing, cutting, or rupture of the cover results in the tissue incompatible ultrasound coupling gel spilling into the body cavity. During ultrasound guided needle biopsy, aspiration, intracavity and intraoperative procedures, sterile covers produced from latex, polyurethane, polypropylene and other polymers, such as described in U. S. Pat. No. 4,593,699 to Poncy et al., U.S. Pat. No. 5,259,383 to Holstein et al. and U.S. Pat. No. 5,676,159 to Navis, are used with such tissue incompatible gel chemicals. A puncturing needle can carry such chemicals into the body, such as into the breast or into amniotic fluid, since gels are present on the skin of the patient at the point of needle insertion, as well as between the transducer and the sheath to accomplish ultrasound acoustic coupling. PA1 5. Solid couplant hydrogels in sheet form that are placed directly upon and remain on the skin of the patient, during the ultrasound exam, such as those described in U.S. Pat. No. 5,522,878 to Montecalvo et al., solid film-coupling gel layer combinations such as disclosed in Echo Ultrasound catalog, Volume 10, page 16 or Cone Instruments catalog, Volume 17, 1996, page 26, and fluids and ultrasound gels placed directly on the patient can create non-uniform layers with wrinkles and/or entrapped air between the transducer and patient which may result in degradation of the acoustic information.
It is an object of the present invention to combine the ultrasound couplant, the protective microbial barrier and lubrication into one homogenous, uniform, integral, solid sheath which is a passive, sterilizable and disposable media that conducts or couples the ultrasound acoustic energy between a transducer and a body.
It is another object of the present invention to provide an ultrasound couplant sheath that is applied to the transducer as a microbial barrier so that the integral solid ultrasound coupling nature of the conformal sheath eliminates the need for a protective cover and separate ultrasound couplant, and blocks the transmission of pathogens or from patient to patient in the routine external non-invasive use of medical ultrasound.
It is a further object of the present invention to provide a solid, conformal sheath that is attached to the transducer and which moves over the exam surface as a part of the transducer, thus providing uniform and reproducible coupling over the entire active surface of the transducer that is in contact with the site being examined.